The invention relates to a method for the internal cooling of a rotating object with liquid gas and to a device for implementation of the cooling method. The method is suitable in particular for the cooling of molded parts in injection molding machines with rotating molds.
In many fields of technology there is a requirement for an internal cooling of a rotating object. Such an internal cooling is achieved by introducing a cooling liquid, for example water, from a non-rotating, fixed object into the at least partially adjoining rotating object. In the rotating object, the cooling liquid is then conveyed to the part to be cooled. The liquid absorbs the thermal energy from the part and takes heat away, thereby producing a cooling effect. In doing so, the rotating object, for example, can be designed as a shaft, the fixed object, for example, as a bearing for supporting the shaft.
The introduction of the cooling liquid from the fixed object into the rotating object usually takes place axially. The transfer point between the fixed and rotating objects is situated on the axis of rotation. Such an arrangement is desirable, because with it the transfer point is not moving relative to the fixed object. There are, however, instances, where the rotating object is not axially accessible.
In some applications it is desirable to cool with a liquid gas instead of with a conventional liquid. Cooling processes with liquid gas are known as such. In them, a liquid gas is conveyed to the part to be cooled, whereby it usually is compressed all the more, the closer it is to the part to be cooled. At the part to be cooled an evaporation and an expansion of the initially liquid, compressed gas is permitted. During evaporation and expansion, thermal energy is withdrawn from the part to be cooled, as a result of which a cooling effect is produced. The gas is then removed in a gaseous physical condition (condition of aggregation).
If such a liquid gas is to be introduced into a rotating object from a fixed object, particular problems occur. The liquid gas possibly-depending on its chemical composition, temperature and pressure-is in a special physical condition (condition of aggregation), which, while advantageous for the cooling, is exceedingly delicate with respect to the handling. In any case, an evaporation and/or expansion of the liquid gas has to be avoided, because this would lead to freezing of the transfer point. Because of the special physical condition (condition of aggregation) of the liquid gas, in particular sealing problems at the transfer point have to be solved.
It is an object of the invention to provide a method for the internal cooling of a rotating object with liquid gas, which solves or minimizes the problems set forth hereinbefore. It is furthermore an object of the invention to create a device for the implementation of this method.
In case of the method in accordance with the invention for the internal cooling of a rotating object, liquid gas is pressed from at least one inlet channel in a fixed object into a ring-shaped groove disposed at a contact surface between the fixed object and the rotating object. From the ring-shaped groove, the liquid gas is pressed into at least one channel in the rotating object and fed to at least one part to be cooled at which point the liquid gas evaporates, absorbs vaporization heat, and is removed as gaseous gas.
In preference, in an area surrounding the part to be cooled a greater cross-sectional surface area is made available to the liquid gas for flowing through, such as the sum of the cross-sectional surface areas of the at least one channel. As a result of this greater cross-sectional surface area in the area surrounding the part, the liquid gas evaporates and expands while absorbing thermal energy. The volume of the gas following the expansion, for example, can amount to 600 times its volume prior to the expansion.
The total cross-sectional surface area, which is made available to the liquid gas on the way from the fixed object to the object to be cooled, is preferably maintained constant or reduced, so that the liquid gas does not expand on the way to the part to be cooled and so that an optimum cooling effect is obtained at the part to be cooled. It is particularly advantageous to reduce the total cross-sectional surface area at least once, which is made available to the liquid gas on the way to the part to be cooled, so that the liquid gas is compressed. This can be achieved, for example, by contractions of the channels. While the inlet channel in the fixed object may have a diameter of several millimetres, the last section of the channel in the rotating object may have a diameter of 0.5 mm or less; even capillary dimensions can be utilized. In this regard, xe2x80x9ctotal cross-sectional surface areaxe2x80x9d is: the cross-sectional surface area of the one channel, if only one channel is present, resp., the sum of the cross-sectional surface areas of all channels, if several channel are present; in this, the cross-sectional surface areas are always measured vertical or perpendicular to the direction of flow of the gas.
The device in accordance with the invention for the implementation of the method has a fixed object, in which an object rotating around a rotation axis is rotatably fixed. The fixed object has at least one inlet channel for liquid gas. The rotating object is surrounded by a ring-shaped groove, into which the at least one inlet channel leads and the center of which is situated on the rotation axis of the rotating object. The ringshaped groove can be machined into the rotating object and/or into the fixed object. The rotating object has at least one channel for liquid gas that leads out from the ringshaped groove into the area surrounding a pan to be cooled.
The area surrounding the part to be cooled preferably has a greater cross-sectional surface area than the sum of the cross-sectional surface areas of the at least one channel in the rotating object, whereby these cross-sectional surface areas are measured in essence vertically or perpendicular to the rotation axis. As a result of this, the gas is provided with sufficient volume for an expansion. The area surrounding the part to be cooled can, for example, be at least one expansion chamber.
The cross-sectional surface area of the inlet channel or, if several inlet channels are present, the cross-sectional surface area of the inlet channels, is preferably greater than double the cross-sectional surface area of the ring-shaped groove. If this requirement is fulfilled, then the liquid gas does not evaporate and/or expand in the vicinity of the ring-shaped groove. Evaporation and/or expansion could have the consequence that too much heat would be removed from the surroundings of the ringshaped shaped groove and that this surrounding area would freeze, which is undesirable.
The double cross-sectional surface area of the ring-shaped groove is preferably greater than the sum of the cross-sectional surface areas of the at least one channel. The sum of the cross-sectional surface areas of the at least one inlet channel to the ring-shaped groove preferably remains constant or is reduced. The sum of the cross-sectional surface areas of the at least one channel leading to the cooling place preferably remains constant or is reduced. By means of such measures, however, the liquid gas is compressed on its path to the place to be cooled, so that an optimum cooling effect is obtained.
In order to prevent the occurrence of accumulations of heat, advantageously at those points where an accumulation of heat could occur, porous steel is utilized. This material stores the cold and, if so required, absorbs thermal energy.
The method in accordance with the invention can advantageously be used for the cooling of molded parts in injection molding machines with rotating molds. Rotating molds like this provide many advantages. There is, e.g., the possibility to inject the molten mass (for example, molten plastic mass) into the mold from several injection stations. With this, first of all molded parts with different geometrical shapes, different colors, or made of different materials can be manufactured. Secondly, with this it is also possible to make molded parts out of several components (multi-component process). Therefore, molded parts that have several colors or consist of several materials (assembly injection molding) can be made. Rotating molds apart from this also make possible the utilization of intermediate stations for different operations and shorter cycle times. The liquid gas can be fed in continuously or else also in batches, e.g., only then, when cooling within an injection molding cycle is necessary.
With the method in accordance with the invention, and with the device in accordance with the invention, cycle times can be even more massively reduced thanks to the exceedingly efficient cooling of just-injected molded parts. The invention presented here solves the problem of the introduction of liquid gas into the shaft rotating from time to time, on which the injection mold is suspended.